Many applications require a particulate solid particle comprising a polymer matrix. Often it is required that the particulate solid has a narrow particle size distribution and a particular desired shape.
For example, in the field of chemically produced toners for electrophotography it is desirable to have particulate solid particles having a narrow particle size distribution so as to provide suitable toners without the requirement for classification. Toner particles which are too big can cause a reduction in the resolution of the printed image and may mechanically damage the components of the printer or photocopier (e.g. scratched photoconductor surfaces). Toner particles which are too small can cause background development problems and may be difficult to clean from the surfaces of the components of the printer or photocopier (e.g. photoconductor). Similarly, toner particles which are too loosely fused or coalesced can mechanically break up, especially during the triboelectric charging process. Toner which is coalesced too much and is too spherical can be difficult to clean from the photoconductor drum if the transfer efficiency is not sufficiently high. Accordingly, for electrophotography the control of toner particle size distribution and shape is highly desirable.
Emulsion association (sometimes emulsion aggregation) is a known process to provide particulate solids (e.g. toner particles). In this process a dispersion of polymer particles along with other non-polymeric materials such as pigments and charge control agents are associated together to form clusters, the clusters are often grown and/or stabilised and then the clusters are heated to coalesce each cluster so forming the final particulate solid.
An example of this kind of approach is disclosed in PCT patent publication WO2009/053688.
We have found that the heating conditions used to coalesce the clusters can be quite severe. For example WO2009/053688 coalesces the clusters at a temperature of 115° C. This is an energy intensive step as the liquid medium is often water which has a high heat capacity. We have found that the heating step tends to destabilise the clusters such that they may continue to aggregate during the heating step. This results in an undesirable broadening of the particle size distribution and the production of a grit fraction of oversized material. Such grit often needs to be removed.
It is possible to use polymer particles wherein the polymer contains the repeat units from ethylenically unsaturated monomers having a hydrophilic group (e.g. hydroxy ethyl methacrylate). This tends to have the advantage of better stabilising the clusters to aggregation during the coalescence step but it can simultaneously inhibit and slow the coalescence of the clusters. This slowed coalescence means prolonged coalescence times and/or higher coalescence temperatures need to be utilised.
Also, when higher weight ratios of non-polymeric materials are incorporated into emulsion aggregation processes coalescence of the clusters become increasingly difficult and higher temperatures are needed.
Some of the components used in preparing emulsion association toners are thermally and/or hydrolytically unstable. For example some polymers, pigments, charge control agents, waxes or magnetic materials can be adversely affected by the prolonged heating step especially in an aqueous liquid medium.
Some components used in preparing emulsion association toners are somewhat soluble in aqueous liquid media at elevated temperatures. In some cases prolonged exposure to higher temperatures can in effect strip components out of the particulate solid by dissolution.
The present invention is directed to address one or more of the abovementioned problems.